3.1: Lung Volumes
- Page ID
- 34419
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Introduction
In this section we will look at some of the nomenclature for a variety of lung volumes and how these are clinically pertinent and can change in disease. We will also begin to look at the work of breathing and what factors affect how easy or hard the lung is to inflate, that is, lung compliance. We will then see how breathing pattern is generated to improve the efficiency of the lung and reduce the work of breathing.
Lung Volumes
First let us look at lung volumes. This trace from a spirometer (figure 3.1) shows the change in lung volume as a patient breathes normally and then performs some specific maneuvers.
Figure 3.1: Lung volumes detected by spirometry.
Let us work through the trace from left to right. The initial part of the trace shows resting or "tidal" breathing. The amount of volume inspired during each breath is referred to as tidal volume.
Once a normal expiration is complete, however, the lung is far from empty, and when instructed, this patient (figure 3.1) breathes out as far as they can; this excess that comes out the lung is referred to as the expiratory reserve volume.
Even at this point, however, some air remains in the lung, and this is referred to as residual volume. Even with maximal efforts, this volume cannot be exhaled, so at no point can the lung be fully emptied. This also means that residual volume can never be measured with a spirometer.
Our patient (figure 3.1) returns to normal tidal breathing for two breaths before taking a full breath in, filling the lungs as much as they can. This extra volume into the lung after a normal tidal inspiration is referred to as inspiratory reserve volume. Related to this volume is the inspiratory capacity, which is the volume that can be taken into the lung after a normal expiration; inspiratory capacity is a useful clinical measurement that we will return to when we deal with some disease states.
Another clinically valuable measurement is vital capacity, which is the volume of air that our patient can move out of the lung after a full inspiration, that is, the total lung capacity, minus the residual volume (remember: residual volume cannot be expelled). Forced vital capacity is a common measure taken in pulmonary function testing, and this is simply the volume that can be expelled from total lung capacity during a forceful expiration. The importance of this maneuver being forced will be dealt with when we look at airway compression (chapter 6).
While the volumes we have just seen measured by spirometry in the pulmonary function lab provide valuable clinical information, we need to now look at some physiological variables that are also critical for our understanding of lung function and disease.
Components of Tidal Breathing
As you have seen, the volume of air inspired during a normal breath is tidal volume, and the size of this is dependent on body size, but in the example here is listed as 500 mL (a good approximation). Not all this 500 mL reaches the gas exchange surfaces in the respiratory zone, however, as some never gets further than the conducting zone (i.e., it stays in the anatomical dead space). From chapter 1 we know that this dead space has a volume of 150 mL, so the amount of air reaching the alveoli in the respiratory zone is our tidal volume (500 mL), minus the dead space volume, so alveolar volume is 350 mL.
This brings us to an important point of clarification. Minute ventilation (denoted as Ve) is the volume of air exchanged in the lung within a minute. This is analogous to cardiac output, the volume of blood pumped by the heart in a minute. As such, minute ventilation is the average tidal volume (VT) multiplied by the number of breaths taken in a minute (RR).
\[Ve = RR \times V_T \nonumber \]
So if respiratory rate is 10 bpm and tidal volume is 500 mL, minute ventilation is 5,000 mL.
\[Ve = 10\, bpm \times 500\,mL = 5,000\,mL\, per\, min \nonumber \]
Physiologically more important, however, is the alveolar minute ventilation (VA) that accounts for the "wasted" ventilation that never reached a gas exchange surface but remained in the anatomical dead space. So the calculation for VA is
\[V_A = RR x (V_T - V_D) \nonumber \]
where VD is the anatomical dead space (approximately 150 mL). So for our previous example, alveolar minute ventilation is
\[V_A = 10 x (500 - 150\,mL) = 3,500\,mL\, per\, min \nonumber \]
describing only the volume of air that reached the respiratory zone.
So far the involvement of anatomical dead space might seem academic, as it remains constant. But let us consider a different breathing pattern (as often occurs in disease states).
In our example above minute ventilation is 5,000 mL, but accounting for dead space we see that alveolar minute ventilation is 3,500 mL. Now let us consider another breathing pattern—one typical of a patient with restrictive lung disease where tidal volume is reduced and respiratory rate is increased. With a tidal volume of 250 mL and rate of 20, the minute ventilation remains the same, 5,000 mL.
\[Ve = 20\,bpm x 250\,mL = 5,000\,mL\, per\, min \nonumber \]
But calculating alveolar minute ventilation we see that a greater proportion of the reduced tidal volume is consumed by dead space.
\[V_A = 20 x (250 - 150\,mL) = 2,000\,mL \,per\, min \nonumber \]
So despite maintaining the same minute ventilation, the second patient’s alveolar minute ventilation is reduced by 1,500 mL, which is significant given that this is the volume of air going to the gas exchange surfaces.
This partially explains why increases in ventilation are initially achieved by increases in tidal volume; as shown in figure 3.2, as tidal volume increases during exercise intensity (represented by oxygen uptake) until it reaches a plateau. Only when this plateau is reached are further increases in minute ventilation achieved by increasing respiratory rate.
Figure 3.2: Changes in breathing tidal volume and respiratory rate with increasing levels of exercise.
So why not keep increasing tidal volume? At higher lung volumes the elastic limit of the lung is approached, and it takes more energy (muscular force) to expand, so it is more efficient and the work of breathing is less if the rate of breathing is increased to achieve higher levels of minute ventilation. This brings us to our next topic, lung compliance.